The proposed expansion of the BoDean Co. asphalt plant, located on two acres in Santa Rosa, CA is surrounded by residential neighborhoods. Three new 14’ diameter batch silos and conveyor equipment rise more than 85’ above the 40’ x 60’ foundation pad and can contain 900 tons of bituminous material batched at 300⁰ F.
RGH Geotechnical Consultants assigned the project to Site Class D with ground acceleration parameters S s = 1.85g and S 1 = 0.72g. Given the small footprint, a single boring indicated highly variable alluvial deposits and that “the site is underlain by interbedded soft to very stiff clay and loose to very dense gravel and sand”. Two thin layers of liquefiable material were identified at depths of 7 and 15 feet below grade and groundwater was encountered at about 10 feet.The initial foundation design prepared by the silo manufacturers was a 5’-6” thick monolithic slab intended to counteract the high overturning reactions. Differential settlement across the 40’ dimension of the pad was estimated to be as much as 5” and was deemed unacceptable by the Santa Rosa Building Dept. for this seismic zone.
Travis Whitted, Geotechnical Engineer and Vice President of RGH Consultants in Santa Rosa, CA recommended a deep foundation system to overcome the differential settlement problem. Driven piles were not an option in this neighborhood and drilled cast in place piles were rejected due to the high water table, potential caving soils and low skin friction values. Helical piles provided the flexibility to deal with the variable conditions, eliminated off-haul
of spoils and provided direct feedback (continuous monitoring of installation torque) that each pile would perform as designed with minimal proof testing of production piles.
Jeff Martin of Steelhead Engineering in Freedom, CA designed the revised foundation. He explains, “The added cost of a deep foundation is never easy for a client to accept. BoDean was willing to make the investment but wanted a value engineered solution. Fortunately, the consolidation was expected to occur in the upper 15’ and the liquefiable layers were thin so slender piles were our most cost effective option. Using helical piles we were able to achieve the required capacities at whatever depth we encountered dense enough soil. Strikingly, even across this 40’ x 60’ pad installed lengths varied between 25’ and 75’. The cost of the piles was partially offset by drastically reducing the volume of concrete and excavation.”
The foundation incorporated 144 A. B. Chance SS175 helical piles installed to a minimum torque of 8,500 foot-pounds for an allowable load of 42 kips per pile. The high loads required tight spacing so the piles were slightly battered to maintain minimum required spacing between the lead sections at depth.
BoDean’s owner Dean Soiland and Chief of Operations Charlie Young visited the offices of Maloney Construction Inc. in Novato, CA to observe the installation of a typical pier prior to accepting the new design. Sean Maloney, the president of Maloney construction was happy to demonstrate the installation. “Dean and Charlie came over to my yard and watched us install a helical pier. We showed them how the piers are installed, how they work, and how we continuously monitor the torque,” explains Maloney. The men were satisfied and the project went ahead.
Installing the piles and terminations took less than a month, including three weeks of drilling.
The 144 installed SS175 helical piles, each had a minimum 8″, 10″ and 12″ helix lead while several had one or two additional 14” helices depending on the conditions at each location. The pile terminations were only 15” apart in many locations and so were installed at a 3° to 6° batter to maintain adequate spacing of the lead sections. A John Deere 120 excavator with a 12,000 ft*lb torque motor was used for installation.
“Just to show how variable the soil conditions really were, a few piles reached torque requirements at 25 feet–others at 75 feet,” says Maloney. “As we drive the piles, we record the torque every foot with a strain gage torque monitor to make sure that we have the required embedment and torque before we call the pile good.” explains Maloney.
Since the required quantity of SS175 extensions was unknown, timely resupplying was critical and a close working relationship with Pacific Helix Distributing was instrumental. “Bill Robowski worked with us to get materials on-site when we needed them” points out Maloney.
Each helical pier was terminated with a DYWIDAG adapter and 150 ksi rope thread bar to allow for a positive mechanical connection to the back-to-back channels that were used as primary reinforcing steel and the 1-1/4” anchor rods specified by the silo manufacturer. Concrete volume was minimized by
utilizing Geofoam blocks to create a waffle slab with a 12” minimum slab thickness and an overall maximum depth of 2’-6” at the integral grade beams.